A hydrometer is a scientific instrument used to measure both the specific gravity and the density of a liquid, based on Archimedes’ principle of buoyancy. It is composed of a sealed glass bulb with a long skinny tube connected to one end, and it is weighted with mercury or lead shot under the bulb so it can float upright. The tube portion is labeled with a scale so when it is placed in a liquid, the measured specific gravity can be seen. The instrument is typically calibrated for use with liquid at a certain temperature, and the scale on it can vary depending on its intended use.
Archimedes’ principle of buoyancy states that an object floating in liquid is lifted up by a force equal to the weight of the liquid being displaced by the object. This means that the lower the density, the more the hydrometer will sink in the liquid sample. On the other hand, with higher density, there is greater force that will lift the bulb higher in the liquid.
In order to use a hydrometer, the liquid sample should be poured into a tall glass cylinder or similar container. The hydrometer is placed vertically in the liquid with the bulb end down so it bobs and floats in the sample. The depth of the bulb flotation is measured using the scale on the neck of the tube, and the reading should be taken at the plane of the water level, not where the liquid clings up the side of the neck. The scale is typically measured in terms of specific gravity, with a range from just below 1 to just above it, such as 0.95 to 1.05.
The specific gravity of a liquid is the ratio of that liquid’s mass to that of an equal volume of pure water. In simpler terms, it is how many times lighter or heavier the liquid is than water. Since it is a ratio of mass, specific gravity has no units. For example, a specific gravity of 1 means that the liquid has the same mass and density as the same volume of pure water, or if the measurement is less than 1, the liquid is less dense with a lower specific gravity.
Since the density of a liquid generally changes with temperature, a hydrometer is typically calibrated to be used with liquid at a certain temperature. Traditionally, a hydrometer was calibrated to measure specific gravity using the density of water at 39° F (4° C) because that is the point at which pure water reaches maximum density. Recently, however, it is more common for one to be calibrated to 60° F (15.5° C) or 68° F (20° C). The calibration of the instrument will usually be labeled on the neck of the tube, and the liquid sample should be at the calibration temperature when the measurement is taken.
There are several different types of hydrometers, which have different scales that vary depending on their use. A hydrometer for heavy liquids will usually have a scale starting at 1 and above, and one for light liquids might have a scale starting well below 1. A battery hydrometer, which often comes as a digital model, measures the specific gravity of battery liquid and can be used to determine the condition and charge of a battery. Other hydrometers include alcoholometers, which measure in terms of proof of alcohol, and saccharometers, which measure the density of sugar in a sugar solution.